1. Field of the Invention
This invention relates generally to desiccant-assisted air conditioning systems and processes, and more particularly to an air conditioning system utilizing a compressor, a condenser coil, an evaporator coil, supplemental desiccant coils, and damper and valve arrangements that direct air and refrigerant through the system in several different thermodynamic operating paths and cycles for significantly improved efficiency and energy conservation.
2. Background Art
The control of humidity in indoor environments plays a very important role in providing indoor air quality. Reducing the volume of moisture indoors can reduce the growth of microbiological organisms such as mold, mildew and bacteria, which require moisture to thrive. Airborne contaminants are also often carried with the moisture in the supplied air streams. Most conventional air conditioning processes and systems do not effectively control humidity, nor provide adequate delivery air conditions, in anticipation of the various changes and demands of the indoor or outdoor environments. Although conventional systems provide dehumidification, it is an uncontrolled byproduct of its evaporator coil cooling process, and results in the inadequate control of humidity, and excessive energy consumption, and can also result in building and or space content damage.
In the refrigerant compression closed cycle of the conventional air conditioning system, a compressor compresses refrigerant gas to increases its pressure and temperature, in an isentropic adiabatic process. The refrigerant is then passed through a condenser coil where the superheated compressed refrigerant dissipates its heat to the crossing air stream condensing the refrigerant into a high-pressure liquid, which then flows through a metering device or expansion valve that restricts the high-pressure liquid and creates a reverse refrigerant adiabatic effect, after which, the refrigerant is discharged or suctioned to an evaporator coil at lower refrigerant temperature and pressure conditions, which enable the evaporator coil to absorb heat from the crossing air that is forced through the coil by the evaporator fan. The air exiting the evaporator coil is discharged as cool air and the refrigerant absorption process changes the refrigerant from liquid-gas to gas, which is then suctioned back to the compressor to complete the closed cycle. Increasing the refrigerant conditions of the evaporator, or lowering the condensing refrigerant temperature and pressure improves the compressor and system performance and energy consumption.
In the air cooling process, the conventional finned evaporator coil provides dehumidification only if the saturated vapor conditions are achieved in its crossing air, and additional cooling is typically necessary to augment moisture removal. This accomplished by lowering the refrigerant pressure and temperature by increasing the compressor capacity or lowering the crossing air stream volume in the evaporator. Efficient heat transfer of a coil is dependent upon the temperature differential between the refrigerant temperature relative to the temperature of the crossing air. The accumulation of water on the evaporator fins serves as an excellent conductor for transferring heat energy to its crossing air stream. The temperature of the water on the fins tends to become lower quickly, because of its direct conductive energy exchange, and at lower temperatures it consequently crystallizes and freezes; it becomes an insulator and diminishes energy transfer capabilities and effectiveness. The ice build can also restrict the air path and further diminish the conductive thermal energy transfer capabilities and efficiencies of the refrigerant.
Thus, if frost becomes a problem, the system requires sequencing to a defrost mode, which stops the refrigeration cooling effects. Added heat energy is often required to accelerate the ice melting effect, or depending upon the temperature of the crossing air, the air itself may be utilized to defrost the accumulated ice. Defrosting or non-continuous cooling can adversely affect the air quality and/or comfort level in the conditioned space. Additional cooling is needed to compensate for any added heat provided by the defrost process and circumstances.
A conventional heat pump also utilizes finned coils and operates on the same principle as an air conditioning system, except that it provides a reversing valve and other controls that reverse the refrigerant flow between the evaporator and condenser coil so that outdoor heat exchanger coil becomes the evaporator and the indoor coil becomes the condenser. This enables the suctioned refrigerant to absorb the remaining heat from the outdoor air and the compressed refrigerant to dissipate its heat at the indoor coil which then heats the conditioned space through its crossing air stream. In the heating mode, the refrigerant cooling cycle takes place through the outdoor coil. At low outdoor temperatures, frost tends to build on the finned coil and lessens the system efficiencies, as previously described; a defrosting mode to remove the frost build up becomes necessary, which is accomplished by re-reversing the refrigerant flow.
Desiccant assisted air conditioning systems are also known in the art, which typically incorporate a rotating desiccant wheel that rotates between two air streams to provide dehumidification or humidification by alternating the energy in a gas phase change process. In such systems, the air (process air) delivered to the interior of a space to be conditioned space crosses the desiccant material, which attracts and holds moisture. As the desiccant wheel rotates, the moist desiccant material enters the regeneration air stream where it is heated to release moisture, which is then vented away. Because humidity is a function of vapor pressure, desiccant materials have the ability to remove or add moisture adiabatically; a reversible thermodynamic process in which the energy exchanges result in substantially constant enthalpy equilibrium. The total desiccant open cycle is somewhat similar to a refrigerant vapor-compression cycle. In a desiccant and air system the heated regeneration air adds energy to the moistened desiccant in a de-sorption process and releases moisture in the regenerating crossing air stream in an adiabatic cooling process. When the desiccant rotates to the process air stream the pre-conditioned desiccant enables the sorption of water and dehumidifies the crossing process air. Adiabatic re-heat then is released in the air stream and completes the desiccant vapor-compression open cycle.
Mathiprakasam, U.S. Pat. No. 4,430,864 discloses a hybrid vapor compression and desiccant air conditioning system utilizing an air thermodynamic cycle for simultaneous removal of the sensible and latent heats from the room return air. The system employs a pair of heat exchangers having a desiccant material thereon, which replace the conventional condenser and evaporator. The refrigerant, room and outside ambient air flows are selectively routed to the heat exchangers to allow one heat exchanger to operate as an evaporator to effect cooling and drying of the room return air while the other heat exchanger acts as a condenser of the refrigerant and regenerates the desiccant material thereon. The heat exchangers are switchable between evaporator and condenser modes allowing for continuous conditioning of the room return air.
The desiccant coils in U.S. Pat. No. 4,430,864, provide a somewhat effective conductive energy transfer to occur, but the desiccant serves primarily to accumulate water. The process re-uses the condensing energy to regenerate its desiccant, which slightly benefits the refrigeration cycle and performance by allowing refrigeration absorption to accelerate and augment some dehumidification in the crossing process air of the desiccant coil. However, the transferable energy provided by the desiccant upon switching is far from being maximized. The pre-wetted desiccant coil upon switching provides a total cooling effect, but most of its interchangeable energy merely replaces what a conventional condenser can already do effectively. Very little refrigerant adiabatic cooling effect is added to augment the compressor performance. The same is true in the process air stream. The pre-dried desiccant merely replaces what a conventional evaporator coil can already do effectively, and is still dependent upon high refrigerant temperature and pressure conditions for the removal of sensible and latent energy in its air stream. The amount of absorbed refrigerant energy from the pre-dried desiccant and crossing air is the direct result of the total average coil temperature and vapor-pressure conditions of its desiccant and crossing air. The total coil average temperature and the average regeneration refrigerant energy transferred to the desiccant is definitely not maximized and the pre-dried desiccant condition elevates very little in proportion to the total average refrigerant conditions and results in a less effective refrigerant adiabatic cooling effect in the refrigeration cycle to augment the compressor efficiency. In the coil switching process, the inefficient total coil average temperature can produce a situation where the regenerated desiccant has insufficient dryness and acts as a heat sink in the process air stream, which results in re-heating the crossing process air and wasted heat energy. A system with only two desiccant coils that replace the conventional evaporator and condenser is also disadvantageous in that it does not provide steady constant air delivery conditions when switching the coils.
Dinnage et al, U.S. Pat. Nos. 6,557,365, 6,622,508, 6,711,907, Published Patent Application 2004/0060315, and Published Patent Application 2005/0050906 disclose systems utilizing rotary desiccant wheels, and utilizing rejected condenser heat as energy to regenerate the desiccant. In general, the basic refrigeration system incorporates part of the condenser coil in the regeneration air stream prior to the desiccant wheel and the evaporator coil prior to the desiccant wheel, in the process air stream. The refrigerant energy is re-used to regenerate the desiccant and the evaporator provides refrigeration capacity and conditions the process air prior contacting the wheel. As with most desiccant wheel systems, this process has limitations in effective cooling. The regeneration entering air is low in temperature, and the vapor-pressure conditions are provided to the desiccant externally in a gas phase change process, rather than heating it directly by the internal refrigerant. Energy is also consumed by continuously rotating the desiccant wheel.
Forkosh et al, U.S. Pat. Nos. 6,487,872, 6,494,053, 6,546,746, Published Patent Application 2004/0112077, and Published Patent Application 2005/0211207 disclose dehumidification and air conditioning systems utilizing liquid desiccants. Dehumidifying systems based on liquid desiccants dehumidify air by passing the air through a tank filled with desiccant. The moist air enters the tank via a moist air inlet and dried air exits the tank via a dried air outlet. In most liquid desiccant systems, a shower of desiccant from a reservoir is sprayed into the tank and, as the desiccant droplets descend through the moist air, they absorb water from it. The desiccant is then returned to the reservoir for reuse. This causes an increase in the water content of the desiccant. Water saturated desiccant accumulates in the reservoir and is pumped therefrom to a regenerator unit where it is heated to drive off its absorbed water as vapor. Regenerated desiccant, which heats up in this process, is pumped back into the reservoir, for reuse. Since the water absorption process leads to heating of the air and the regeneration process heats the desiccant, substantial heating of the air takes place during the water absorption process.